The present invention generally relates to a vibration type transducer for measuring a density or a pressure of a fluid brought into contact with a mechanical vibratory member by detecting a resonant frequency of a vibration system including the mechanical vibratory member, and more particularly, to a structure of a transducer suitable for measuring the density or the pressure of the fluid flowing through a conduit line.
The present applicant has proposed a type of transducer (see Japanese Patent Publication No. 239228/1985) composed of a vibratory member, a cavity formed above at least one surface of the vibratory member so as to be opposite this surface and a tube for leading a fluid to be measured into this cavity, this transducer serving to measure the mass, i.e., the density, of the fluid within the tube on the basis of a variation in resonant frequency of a mechanical/acoustic vibration system consisting of the vibratory member, the cavity and the fluid existing in the tube.
The vibration type transducer of this kind is, however, attended with the following characteristics. In the case of measuring the density of the fluid flowing in the conduit line or the like, exciting forces with a wide range of frequencies act on the vibratory member due to deviation of a flow or disturbances of pressure such as a swirl or the like generated in the tube. As a result of this, an oscillation frequency loses stability, and the error in measurement is thereby increased. For this reason, a measuring limit of the above-described transducer comes to a flow-velocity of several m/sec at the most. The measurement is impossible in a range of higher flow-velocities beyond the foregoing limit.